1. Field of the Invention
This invention pertains to autonomously guided devices employing aperiodic discrete proportional navigation. More specifically, this invention pertains to a guided projectile in the shape of a right cylinder employing spin about its longitudinal axis for gyroscopic stabilization and circumferential explosive impulse thrusters for propulsion, and a method for guiding same.
2. Description of the Prior Art
The general application of aperiodic discrete proportional navigation has been established for some time. The theoretical foundations of proportional navigation were first revealed in Soviet Technical publications over four decades ago, and began to appear in open technical publications in the United States shortly thereafter. Subsequently, they have been widely adapted to commercial and military guidance applications, including virtually all precision guided weapons around the globe. Various theoretical formulations of proportional navigation have been put forward in open literature, including both analog (continuous sensing and control) and discrete (discontinuous sensing and control) proportional navigation. The particular manifestation of the generic proportional navigation principle which is referred to as "discrete proportional navigation" provides a generic, theoretical framework within which many guidance systems mechanizations including that of the present invention are founded.
Simply stated, discrete proportional navigation is defined as discretely induced adjustments to the device velocity components, based on sensed changes in relative attitude to an approaching object or target, which permit a device to achieve an accurate, fixed relative orientation to, and intercept with, that approaching object. In its two basic variations, the designer may choose to either a) vary the magnitude of periodically applied thrusters (period variant); or to b) vary the time intervals between application of fixed magnitude thrusters (aperiodic variant). The generic aperiodic variant of discrete proportional navigation is often selected because of certain intrinsic advantages.
Low cost, extended storage life, and packaging advantages characteristic of fixed magnitude solid propellant thrusters are known and have led to broad application in a host of guided system control applications. Because of the high shock level associated with the firing of each solid propellant thruster, the thrusters are generally rigidly mounted into the primary device structure. This avoids having to otherwise oversize any associated gimbal drive assemblies to accommodate intermittent high torque moments. Body fixed discrete thrust control is a generic attribute associated with virtually all applications of solid propellants for guided system control. An example may be found in U.S. Pat. No. 4,674,408 by Lothar Stessen.
The prior art teaches the method of body fixed sensing of an external approaching object. To implement any form of proportional navigation, it is necessary for the guided device to incorporate some form of external object sensing. The particular sensor technology commonly employed in such applications includes visual spectra, infrared spectra, millimeter wave and microwave radar, among others. In continuous proportional navigation, regardless of the sensor technology being employed, the external object sensor is most commonly mounted in a tracking gimbal assembly in order to permit gimbal rate gyros to measure angular rates corresponding to the external target's relative movement. In either the periodic or the aperiodic form of discrete proportional navigation, the necessity to measure external target relative angular rates is removed, since the guidance principle is based instead on introducing thrusting only when cumulative changes in the relative angle exceed a threshold. Accordingly, gimbal rate measurements are no longer required, provided that body coning motion is successfully removed from measured relative angle changes.
Furthermore as previously established if aperiodic discrete proportional navigation using body fixed solid propellant thrusters is to be incorporated, regardless of the sensor technology being employed, the external object sensor will be subjected aperiodically, to high torque moments, if the sensor is gimbal mounted. The necessity to overcome the gimbal drive assembly inertia would lead to greater device size and possibly higher cost. For these clear and compelling reasons, guided device applications of aperiodic discrete proportional navigation using solid propellant thrusters has commonly incorporated both the external object sensor and the solid propellant thrusters directly into the primary structure of the device. An example of a spin stabilized body fixed sensor can be found in U.S. Pat. No. 4,560,120 by Crawford et al.
Such devices having body fixed sensors typically require some form of an inertial reference system to measure and correct for the changes in the rotational motion of the device from acceleration and deceleration due to the thruster system and precessional error due to the "wobble" of the guided device in flight. Prior to the present invention various approaches to compensate for the spin error and the precessional error were attempted. One known method was to disregard the errors and to rely on the accurate initial placement of the guided device with respect to the external object, such that only a few solid propellant thruster firings would be required to position the device. This design approach was subsequently abandoned as an unrealistic approach. Another design approach has been to incorporate a strap-down inertial system which continuously senses the deviation of the device body about an established reference rim using gyroscopic (inertial) components. See e.g. U.S. Pat. No. 4,676,456 by Grosso et al. Although the performance provided by this approach has been acceptable, failure to meet realistic costs, size and weight goals has been a significant problem.
Finally a design approach was attempted utilizing balanced guided device moments of inertia, i.e. 1:1:1, together with passive and active device balancing features that theoretically would result in entirely eliminating precessional error. Because of the relatively narrow gyrodynamic stability envelope for such a system, and the consequent prohibitive cost of the manufacturing and balancing tolerances that would be required to actually make this approach practical, a moderately large, but slow precessional motion is actually experienced. The residual precessional motion remains large enough to require the incorporation of active deprecessional torquing to bound the magnitude of precession experienced and to incorporate gyros to measure residual precessional biases.